1. Field of the Invention
The present invention generally relates to automated coal mining, tools, equipment, and methods, and more particularly to using force measurements obtained from individual bit picks in bit blocks on continuous and longwall mining machine coal cutterhead drums to reduce the generation of coal cleaving fines, to automatically calibrate earth-penetrating radars, and to predict the best orientations, lacing patterns, and positions for replacement bit picks and cutterhead drums at a next service interval.
2. Background of the Invention
The coal fines and dust produced by continuous and longwall mining machines in underground coal operations is hazardous for a number of reasons. So the reduction and control of such coal fines and dust can go a long way towards improving coal mining safety and the health of miners. One obvious way to control coal fines and dust has been to spray them and the cutting faces with water. So it is common practice now to provide water passages in cutterhead drums and bit blocks, and spray nozzles. The water also helps to cool the machinery, the cutting face, and to quench any tendency to spark or burn.
The angles and forces with which the bit tips strike and re-strike the coal face to cleave off blocks and sheets of raw coal have an effect on the amount of coal fines and dust produced by continuous and longwall mining machines. Too much force will crush and pulverize the coal at the points of impact. Subsequent strikes that do not follow the original fractures produced by the initial strikes wastes machine energy and reduces the flaking size. Such can also increase the wear and tear experienced by the equipment.
The Colorado School of Mines and the 20-Mile Coal Company studied this problem and published their paper, Results of Practical Design Modifications for Respirable Dust Reduction on Continuous Miners in Underground Coal Mining, by Brian Asbury, et. al., at the SME Annual Meeting, Feb. 24-26, 2003, Cincinnati, Ohio. They report that about a third of all coal production in the United States comes from underground mines. Essentially all underground coal production is produced by continuous miners and longwall mining machines. Much of the respirable dust produced by cutting drums is generated by material that is crushed directly under the individual bits on the cutterhead. They say that the tip angle, angle of attack, bit penetration, and other cutting geometries all affect the volume of fines and dust that will be generated under an individual cutter bit. Reducing the number of cutters engaging the rock by increasing the bit spacing will also reduce the total amount of fines and dust generated.
FIG. 1 repeats and adds to what was presented by Brian Asbury, et. al., in their Paper. The cutting direction is lateral to the exposed face of the coal. The cutter bit will present a force normal to the face and a cutting force lateral to the face in the cutting direction. Such cutter bit will have a rake and clearance angles relative to the cuts. The depth of cut is “d”. Fractures will radiate from a crushed zone. Each strike of a cutter bit advances the cleaving of material off the face.
If a next cutter bit strikes the propagating fractures in the butt-face cleat coal structure, coal fines can be reduced. The Paper suggests that fines are produced in the crushed zone. If a next cutter bit manages to strike on the propagating fracture, the crush zone will not be as large. Coal fails at approx 2500 psi. The Paper contends that fines can be reduced by reducing the number and increasing the separation between bit blocks. But no mention is made of the coal butt-face cleat structure. In the pick and shovel days, the pick force vector was directed into the vertical face cleat, enabling a peeling off of blocks of coal. Picking squarely into the butt cleats seemed to absorb the energy better and not break.
A coal shearer travels along the face of a coal seam with a large drum or cutting head, slicing off slabs of coal with dozens of replaceable picks. Shearers use different types of drums, and the picks are available in a wide variety of styles, designs, uses, and materials. A conventional pick-flushing drum sprays water through nozzles mounted on top of the cutter drum vanes, e.g., pick face flushing (PFF), and pick back flushing (PBF). The sprays wet the dust particles as soon as the picks strike the coal and cut it. The water naturally gets mixed in with the cut coal as it is discharged from the drum.
Water is often added to the tires of mine sweeping vehicles. Such significantly reduces the available explosive energies by absorbing the energy in the heat of fusion process by changing the phase state of water to gas. If a hydrocarbon (methane gas) ignition occurs in a coal mine, especially when cutting into sandstone boundary rock, water can be placed to absorb significant amounts of the ignition energy.
Shearers are used that can cut coal in unidirectional and bidirectional patterns. Unidirectional cutting is the most common type of cutting pattern used in longwall mining in the United States. The shearer moves from tailgate entry to headgate entry. A leading drum is raised to cut coal while a trailing drum cuts the floor coal. Cleanup is done on each return trip back along the face. In bidirectional cutting, a web width of coal is cut in both directions of travel. Each pass uses two-faced end operations to turn the machine around.
Coal shearers use conveyor belt systems to transport the cut coal away from the face after it is cut. Armored face conveyors move under the shearers to collect the coal as it is cut. Three types of coal shearers are used in longwall mining. Double-ended ranging drum shearer (DERS) can extract coal from seams 58-156 inches thick. When the seam thickness exceeds 60 inches the entire coal seam can be cut in one pass of the machine and in either direction of travel enabling a higher level of productivity and shorter roof exposure time. Two single-ended ranging drum shearer (SERS) machines are used simultaneously on longwall faces with a 60 inch mining height. Two single-ended fixed drum shearer (SEFS) machines are used simultaneously on longwall faces to extract thin coal seams of a 48 inch thickness.
Philip W. Southern thought the solution to controlling dust in longwall coal mining was to eliminate the unnecessary sharp edges in the construction of the bit block mounts and vanes on the cutter drums. U.S. Pat. No. 5,230,548, titled LONGWALL CUTTER DRUM HAVING REDUCED PROPAGATION OF DUST, was issued Jul. 27, 1993. A cutter drum is described that presents smoothed surfaces that will not create dust when falling pieces of coal hit them. The claim is made that the new drum requires less energy to operate because the resistance to rotation is reduced.
The “technical root cause” of lung disease reported in the Upper Big Branch accident investigation was the HS boundary detection technology gap, that if crossed, would have prevented the rotating drum picks “blind” cut into seam boundary rock of the undulating and faulted coal seam. Undulations in the coal bed horizon are influenced by differential compaction of meandering paleochannels, faults, dykes, and other types of depositional anomalies. The point of sensing must be in-real time (i.e., on the cutting drum) and not delayed in time by sensing on the body of the mining machine.
To avoid cutting into the seam boundary rock caused by a meandering paleochannel seam role, the machine automated cut algorithm must begin with the downward floor cut ahead of the paleochannel margin to safely mine through a differentially compacted seam anomaly. The “last cut memory” horizon control algorithms are not useful in automated control of the shearer in undulating environment.
In geologically disturbed environments, the coal shearer operator needs to be to see the machine. Thin layers of uncut roof coal are used for ground control to prevent rock fall into the mining face. The thin seam boundary coal layer is contaminated with biochemically reduced forms of heavy metals.
For example, Felish Peng found 25 to 680 pounds/1012 Btu in boundary layer channel samples, which compares to 6.1 pounds in the body of the seam. Of course, splay-deposits may restart contamination in mid-seam. Heavy metal contamination and silica in respirable dust are drivers in lung disease. Researchers now realize that lung disease causes may include the heavy metals as well as silica dust reactions that diminish lung tissue oxygen transfer into the blood stream.
A serious environmental threat to the industry is the promulgation of rules declaring boiler fly ash toxic because of the re-oxidation of heavy metals in the combustion process. Because of boundary ground control and contamination issues, synthesized picks detection is of value only in interface detection. From a health and safety point of view, thin contaminated boundary coal layers should be left behind in the mine.
David Chang determined that a small gap in a loop of wire will exhibit a resonant driving point impedance (Z1N) that depends on the thickness of any slightly conducting Earth dielectric positioned next to the loop antenna. Chang and Wait formulated equations governing such gap impedance which is a complex variable satisfying the Cauchy-Riemann conditions.
The conventional GPR antenna distance to the coal rock boundary is oftentimes less than 0.2 meter. The radar electromagnetic (EM) wave field components roundtrip travel time is less than two nanoseconds that creates a detection and interpretation problem for conventional GPR. The circuital wire loop, which would never withstand the drum-cutting environment, was replaced with a resonant microstrip patch antenna (RMPA). Dr. Peter Petrov and his colleges formulated the EM equations and computer code to predict the RMPA impedance change with a varying and slightly conducting Earth dielectric. As it turns out, the theoretical physics of Maxwell, Heaviside, and von Hippel was required to combat the effects of the rotating picks fracturing of the thin coal boundary layer and water spray changing the dielectric constant of the slightly conducting coal layer.
To combat the state-of-the-art interface varying dielectric calibration problem, new radar technology was developed and in-mine demonstrated in Consol and the Oxbow coal mines. This work solved the problematic geologic clutter (e.g., rapid spatial change in dielectric constant) and very high reflection in the “early arrival time” that obscures “late arrival time” EM field components reflected from air or water filled entries.
The double sideband gradiometer (DSBg) ground penetrating radar (GPR) technology was developed on a highwall mining machine and horizontal drill string navigation collar enabled distance determination relative to the seam boundary. Our early DSBg GPR work resulted in a validated prototype.
The SCARE functionally was developed to suppress the clutter and reflection from the pick fractured coal layer and detect the EM field components reflected from the coal-rock interface and even a gradational boundary. The double sideband gradiometer (DSBg) GPR with SCARE functionality suppresses early arriving EM wave field components (ER1) by up to seventy dB relative late arriving field components (ER2) from the coal-rock boundary interface. The suppression depth can be selected by the adjusting the modulation frequency, which is one-half of the sideband frequency separation. The fracture suppression depth is set to approximately the pick length. By including a bit force vector sensor in the bit block, the interface pick intersection can be determined. It's used for automated calibration, controlling the rock cutting depth when required, and to acquire data for bit lacing for minimum fines production.
Sometimes it is necessary to cut through the contaminated coal layers and into the boundary rock, e.g., for machine clearance. Bit blocks with piezoelectric sensors can be used to measure the bit tip force vectors and determine bit contacts with the rock interface. The rock cut depth can thereafter be accurately and automatically controlled by the machine.
Dust produced by cutting drums is generated by rock and coal that is crushed directly under the individual bits on the rotating cutter head. The pick tip angle of attack, bit penetration, and drum lacing pattern affects the fines and dust generated. A pick bit block, instrumented with piezoelectric sensors, was developed and trialed in a pick cutting force measuring apparatus. The objective of the work was to measure the pick vector force angle in real time. Reducing the number of picks on the drum and increasing pick bit bock spacing was suggested by the CSM work to reduce fines and dust.
There are a number of conventional devices and methods that claim dust and fines reduction (e.g., very smooth drum surfaces and shaped picks). Coal fracturing is dependent on the cleat geometry and microfractures induced by horizontal platonic forces relative to the vector pick force angle and propagating fracture angle. Reduction in coal fines occurs when the next pick intersects the propagating fracture. Measurement of vector pick angle and uncut coal thickness data can be transmitted by radio modem to the shearer.
The uncut coal DSBg GPR with SCARE functionality will look up to detect the condition of roof support canopy loss of hydraulic pressure and look forward to detect abandoned hydrocarbon well casings. The multiple-mode smart coal cutting drum data will be transmitted from the rotating drum to the mining machine and processed. The acquired data will enable optimization of coal cutting for minimum fines generation.